[clang] [CMake] Change GCC_INSTALL_PREFIX from warning to fatal error (PR #85891)

[Fangrui Song via cfe-commits]

MaskRay wrote:

> @MaskRay can you please make the error message more informative? It should probably say that users additionally have to set `RUNTIMES_CMAKE_ARGS=...` as well as generating `clang.cfg`, `clang++.cfg` config files. (And what about `flang.cfg` and/or `flang-new.cfg`?)

I haven't used `RUNTIMES_CMAKE_ARGS=` myself. The purpose of the message is to provide brief information to help users to find a solution. Covering everything is probably out of scope... If they find this PR and your comment, it will have provided sufficient information.

I tried grepping `RUNTIMES_CMAKE_ARGS` and did not get many results. The documentation can be independently improved.

https://github.com/llvm/llvm-project/pull/85891